The present invention relates to a wiper unit controller and a method for controlling a wiper unit.
Automobiles are equipped with wiper units for improving visibility when driving in the rain. A wiper unit includes a lever, which is located near the steering wheel. The lever is operated to activate a wiper switch and select one of an off position “OFF”, an intermittent wipe mode position “INT”, a low speed wipe mode position “LO”, and a high speed wipe mode position “HI”. As a result, a wiper of the wiper unit wipes the windshield glass in the selected mode. A knob is located at the distal end of the lever. When the lever is arranged at the intermittent wipe mode position “INT”, the driver rotates the knob so that the wiper performs intermittent wiping at a desirable cycle, which is in accordance with the rotational position of the knob (refer to, for example, Japanese Laid-Open Patent Publication No. 10-315919).
In the intermittent wipe mode, a variable resistor (volume) is used to reflect the rotational position of the knob in the intermittent wipe cycle. That is, when the knob is rotated during the intermittent wipe mode, the resistance value of the variable resistor varies accordingly. A controller detects the resistance value of the variable resistor and determines the intermittent wipe cycle in accordance with the resistance value.
More specifically, with reference to FIG. 5, a wiper unit 101 includes a fixed resistor 111 and a variable resistor 112. The fixed resistor 111 and the variable resistor 112 are connected in series between positive and negative terminals of a constant voltage power supply. The fixed resistor 111 and the variable resistor 112 divide the voltage output from the constant voltage power supply to generate divided voltage V, which is input to a controller 120. The voltage V input to the controller 120 may be expressed by the next equation when the voltage output from the constant voltage power supply is represented by Vcc, the resistance value of the fixed resistor 111 is represented by R1, and the resistance value of the variable resistor 112 is represented by Rv.V=Vcc×Rv/(R1+Rv)
From this equation, Rv may be expressed by the next equation.Rv=R1×V/(Vcc−V)
Accordingly, by storing the values of R1 and Vcc in the controller 120, these values may be used to enable detection of the resistance value Rv of the variable resistor 112, which is in accordance with the rotational position of the knob. The controller 120 recognizes the rotational position of the knob from the ratio of the resistance value Rv of the variable resistor 112 relative to the maximum resistance value of the variable resistor 112. The controller 120 then determines the intermittent wipe cycle in accordance with the recognized rotational position of the knob.
To determine the intermittent wipe cycle in this manner, the controller 120 must store the maximum resistance value of the variable resistor 112. However, the maximum resistance value of the variable resistor 112 has a relatively large tolerance (for example, ±30% relative to a reference resistance value). Thus, the maximum resistance value varies between different variable resistors. Therefore, when the reference resistance value, or ideal maximum resistance value, of the variable resistor 112 is, for example, 1000Ω, the controller 120 stores a value corresponding to a variance of −30% (e.g., 700Ω) as an initial value of the maximum resistance value. If a resistance value Rv that is greater than the stored maximum resistance value is obtained through the above equation, the controller 120 stores that resistance value Rv as a new maximum resistance value. In other words, the controller 120 renews the maximum resistance value of the variable resistor 112 so as to accurately recognize the rotational position of the knob from the ratio of the actual resistance value relative to the maximum resistance value. The renewal of the maximum resistance value is repeated so that it approaches the actual maximum resistance value of the variable resistor 112.
When the variable resistor 112 is used under a low temperature environment (e.g., minus 30 degrees Celsius), the detected resistance value Rv may temporarily exceed the stored maximum resistance value. In such a case, the maximum resistance value would be renewed by the resistance value Rv even though it is a temporary value. If the maximum resistance value is once renewed to such a large value, it cannot be renewed to a smaller value thereafter. Accordingly, if the temperature subsequently increases to a normal temperature (e.g., 25 degrees Celsius), the rotational position of the knob is recognized using that stored maximum resistance value (i.e., the value taken under a low temperature and being greater than a value taken under a normal temperature). As a result, an error occurs between the actual rotational position of the knob and the rotational position of the knob recognized by the controller 120. This hinders wiping at an intermittent wipe cycle that is in accordance with the rotational position of the knob.